1. Field of the Invention
This invention relates to a method and apparatus for the descaling and subsequent cold-rolling of metal strip, particularly but not exclusively steel sheet in the form of strip.
2. Description of the Prior Art
In the making of steel strip, there is a descaling step for removing rust (or scale) from the surface of a hot coil and a cold rolling step for reducing the strip to a predetermined thickness. The descaling step has had its efficiency improved by combining a chemical method (e.g. pickling) and a mechanical method. On the other hand, the cold rolling step is exemplified by either a reversing mill for rolling multiple passes reversibly, or by a tandem mill system for rolling in one pass in one direction through a plurality of rolling machines.
Many existing general plants perform the descaling step and the cold rolling step discontinuously. Since the descaling step employs a continuous line passing hot coils sequentially, an expensive welder is arranged at the entrance side of the descaler. In order to prevent interruption of the line during the welding, moreover, a long accumulator (looper) and a long pickling tank are provided. Since, moreover, the pickled coil has to be fed to the cold rolling mill line, the pickled coil is divided again at the exit of the pickling equipment so that it may be sized suitably for the transfer and storage in the factory, until it is taken up for cold rolling. Between the pickling equipment and the cold rolling mill line, there may be a wide coil yard for absorbing the difference between the production schemes of the two equipments. This yard is equipped with facilities for transferring, storing and managing the coils.
An alternative approach has already been practiced using continuous pickling and cold rolling equipment, in which the pickling equipment and the tandem mill are directly connected.
With the aim of obtaining benefit from use of a single multi-pass reversing roll mill, it has been proposed to employ large size coils, composed of a strip having a length several times, e.g. five times, that of the normal transportable coil. JP-B-57-39844 shows a welder for joining short coils, after which the strip is wound into a large size coil. The welding line stops when the large size coil is completed, and this coil is then unwound through a Sendzimir mill. Multi-pass rolling through this mill takes place, rewinding at each end being onto further drums. The final rolling pass leads to rewinding into the small coils, with shears sub-dividing the strip. The function of the large coil is to achieve a high yield and high production capacity of the reversing mill, by minimizing the periods of acceleration and deceleration of the mill and of threading the strip. Yield is also increased. However, this document does not concern itself with the combination of a descaler and a mill.
Another use of such large size coils, for a different purpose, is shown by JP-B-59-52710. This discloses a line having a welder, an accumulator, a descaler, shears and large size coils on a pair of interchangeable drums. A single pass tandem mill is fed from the large coils alternatingly and delivers rolled strip to shears and drums for small coils. The size of the large coil is said to be five to ten times the standard small size. The reason given for the use of the large coil is to allow continued operation of the descaler even when the mill is stopped. There may be many reasons for stoppage of the mill such as roll changing. Thus, the formation of the larvae coils is apparently necessary only in order to deal with such a stoppage of the mill. Since there are no particular restrictions on the capacity of the descaler, the objective in this apparatus must be to maximize the capacity of the expensive tandem rolling mill. This means that the capacity of the descaler must be equal to the desired capacity of the rolling mill, and consequently the throughput speed of the descaler must be at least equal to that of the rolling mill. A large and expensive descaler is required. The capacity of a tandem mill is in principle much greater than that of a reversing mill.
A clear distinction in the prior art exists between the concept of using a tandem mill, where the aim is to achieve maximum use of the very high capacity of the expensive tandem mill, and on the other hand the concept of use of a reversing multi-pass cold rolling mill, whose capacity is much less than that of the tandem mill. The aim of this second concept is to arrange the plant to achieve, in an economical way, a maximum throughput through the reversing mill, together with if possible, high quality of product.
FIGS. 3 and 4 illustrate this distinction by presenting data of the descaling equipment (i.e. the pickling equipment) and the cold rolling equipment prevailing at present in Japan.
FIG. 3 plots the correlation between the nominal production capacity (per month) of the pickling equipment and the line length of each line. These correlations naturally disperse depending upon the layout of the individual equipments and the product mixing ratios. However, as a whole it can be seen that:
(1) a large line length is required for a high production capacity; and PA1 (2) the line will not always become short in proportion to the capacity in an equipment having a low production capacity. PA1 (1) a higher production capacity can be achieved by a larger number of stands of the rolling mill; and PA1 (2) a small production capacity can be realized by a single stand mill. PA1 (1) a production capacity higher than 100,000 tons per month can be achieved by a large-scale pickling equipment having a line length of 200 to 300 m and a large-scale tandem mill having 5 to 6 stands; and PA1 (2) a production capacity lower than 30,000 tons per month can be achieved by a pickling equipment having a line of length 100 m, which is rather long in relation to the production capacity and a single stand reversing mill of the smallest scale. PA1 (3) the pickling equipment requires a scale as large as that of 100,000 tons per month; and PA1 (4) the cold rolling equipment has to adopt a multi-stand tandem mill having a scale as large as 100,000 tons per month because it is required to effect a predetermined thickness reduction in one pass, or a plurality of single stand reversing mills having a low production capacity have to be provided. PA1 (1) The continuous pickling line connects hot coils consecutively, so that an expensive welder is arranged at the entrance of the pickling equipment. In order to avoid interruption of the line during the welding, a long accumulator is arranged together with a long pickling tank, so that the line length is seriously enlarged as a whole. PA1 (2) The pickled coils to be fed to the cold rolling equipment at a subsequent step are made by dividing the strip again so that their size may be suited for transfer and storage in the factory. PA1 (3) It is also necessary to interpose between the pickling equipment and the cold rolling equipment a wide coil yard for absorbing the difference between the production schemes of the two equipments and plant facilities for transferring, storing and managing the coil in that region. PA1 (a) joining a plurality of coils of the strip to form a long strip length, PA1 (b) passing the long strip length through a descaler, PA1 (c) winding the long strip length into a large coil, and PA1 (d) passing the long length strip in a plurality of passes, with reversing, through a reversible multi-pass cold rolling mill to effect cold rolling, with unwinding of the long strip length from said large coil and rewinding thereof between each two passes through the mill, PA1 (e) the steps (a) to (d) being performed in a single apparatus line and the speed of the strip in the descaler being different from the entry speed of the strip in each of the passes through the mill. PA1 (a) sequentially joining a plurality of coils of the strip into a plurality of long strip lengths each comprising strips from a plurality of said coils, PA1 (b) sequentially subjecting each long strip length to the following sequence of steps: PA1 (a) sequentially joining a plurality of coils of said strip into a plurality of long strip lengths each comprising strips from a plurality of said coils, PA1 (b) sequentially subjecting each of said long strip lengths to the following sequence of steps: PA1 (a) a welder for joining a plurality of coils of the strip into a long strip length, PA1 (b) a descaler for continuous passage therethrough of the long strip length directly from the welder, PA1 (c) first coiling means for coiling a first large coil from the long strip length after passage through said descaler, PA1 (d) a reversible multi-pass cold rolling mill for reversibly rolling the long strip length from the first coiling means in a plurality of passes, and PA1 (e) second coiling means for winding a second large coil from the long strip length after rolling by the cold rolling mill, PA1 (f) the first and second coiling means effecting coiling and uncoiling of the long strip length during its reversible rolling in the cold rolling mill. PA1 (f) means for sub-dividing the long strip length when uncoiling it from the second coiling means after its rolling in the cold rolling mill, and PA1 (g) means for winding into individual coils the portions of the long strip length produced by the sub-dividing means.
On the other hand, FIG. 4 presents the distribution of the nominal production capacity (per month) and the number of such cold rolling plants existing in Japan. For example, numeral 1 indicates a so-called "single-stand reversing mill", and numeral 5 indicates a so-called "five-stand tandem mill". For cold rolling plants, as a whole it can concluded that:
Thus, the situation for a descaling and cold rolling apparatus at present is as follows:
However, for a line having a medium production capacity of about 50,000 tons per month between the foregoing capacities (1) and (2):
Thus, the equipment is so redundant relative to the desired production capacity that high and uneconomical investment is required.
On the other hand, the needs for the production and supply of steel sheet materials are not domestic but worldwide, and instead of the large-scale mills which have existed, medium-scale steel sheet production facilities are desired near the markets for the product.
In order to satisfy the above-specified needs, facilities are required for realizing a highly economical descaling and cold rolling method which is compact for the production scale but requires no excessive investment, and apparatus for realizing that method. However, these are hard to realize with the methods of the prior art.
With the existing plants of the prior art, the following problems are present.
On the other hand, the above-specified items (2) and (3) are rationalized in the continuous pickling and cold rolling apparatus proposed in the prior art in which the pickling equipment and the tandem mill are directly connected. However, this apparatus naturally requires large-scale devices such as a large accumulator between the pickling equipment and the tandem mill so that it is effective for the very high production capacity of the tandem mill, but is of excessive size and cost for a plant having a medium production capacity.
Next, as to the cold rolling equipment, the tandem mills described above are suited for the large-scale plant having a large production capacity but not for a plant having a medium production ability.
On the other hand, the reversing mill has a scale suitable for the case in which a small production capacity is to be attained, but it has been thought that the number of such mills has to be increased for a medium-scale production capacity. A proposal is made in JP-A-57-64403 to combine a descaler and a reversing mill in a continuous line, but this results in a large, expensive and impractical plant, whose capacity is actually limited. JP-A-57-64403 describes a plant for continuous descaling and multi-pass reversing mill rolling. A welder joins the strip, before entry to a descaler, from which the strip passes directly to the mill region. In the mill region, there are large strip accumulators whose intention is to allow the reversing mill to operate on the strip, portion by portion with multi-pass reverse rolling, while not interrupting the progress of the descaler. The strip is not sub-divided or coiled in the mill region. FIG. 3 of this document shows that the capacity becomes saturated, or nearly so, with a cycle length (i.e. the unit length of strip which is subjected to three-pass reversing rolling at one time) of 1,000 to 1,500 meters. Such a length is too short to obtain large benefits in the rolling operation. The apparatus also is impractical. Not only are the large accumulators expensive and bulky, but also it appears that their size limits the capacity of the machine. A large increase of accumulator capacity would result only in a small increase of productivity of the mill.
As mentioned above, JP-B-57-39844 proposes an improvement in the use of a reversing mill, but does not disclose a combined descaling and rolling line. Merely to provide a descaling line in front of the mill of JP-B-57-39844 would lead to an expensive plant, employing two welders and requiring space for storage of the coils, after descaling. The descaling apparatus, if placed after the welder shown in JP-B-57-39844, would be stopped when the welder stops, which is highly unsatisfactory for a descaler which should operate continuously to avoid over-pickling of parts of the strip. This stop time of the descaler would mean that the descaler has to be larger in nominal capacity than is required by the rolling mill capacity. Another disadvantage of the process of JP-B-57-39844 is the need to stop welding and winding a large coil on an entry drum during the first pass rolling, which reduces capacity. Yet another disadvantage is the need to stop the mill during its final pass at each time of coil division, which also reduces capacity.